The present invention pertains to subsea containers and, more particularly, to subsea containers that are unmanned and are used to house equipment that is deployed, performs a function, such as seismic data collection, and is retrieved to withdraw the acquired data at a subsequent time.
Data acquisition systems, such as Ocean Bottom Seismometers, referred to as "OBS", are deployed at a location to detect and store seismic acoustic pulses. The ocean bottom seismometers are normally deployed at a location several miles distant from seismic acoustic pulse generators, such as air guns, which may be towed by a marine vessel.
Presently, there are several types of containers in which data gathering equipment, such as seismic data detection systems, are housed for subsea application. These containers usually involve a pressurized housing having an acoustic pulse detector attached, power supply and tape recorder or some type of data storage within. Problems with these containers occur in several areas. First, withdrawing the data from the container usually requires that the unit be opened to the atmosphere. Data is usually retained by a tape recorder, and the tape must be physically removed to allow playback for further processing. A door is thus provided with seals to protect the internal equipment from the corrosive effects of seawater and to prevent shorting of various electrical components. Opening the compartment door frequently, each time the OBS is used, causes wear on the seals provided and can result in seal failure when exposed to the high pressures encountered in subsea use. Replacement of the door seal is a possible solution, but it requires additional man-hours in assuring proper placement of the seals and waste in replacing seals that might not fail.
Maintenance of ocean bottom seismometer units adds another complication to the problem of door seals. A great deal of sophisticated electronic equipment along with the tape recorder is contained in an ocean bottom seismometer. As such, a power supply, normally a battery pack, must be provided. Generally, the power supply is located in the compartment with the recording unit to provide power for the seismic data acquisition system. Not only must the door be opened to physically remove the tapes from a tape recorder, but the door must also be opened to monitor and service the power supply.
A second problem occurs in the deployment and retrieval of ocean bottom seismometers, one which is environmental related. When an OBS unit is used in an area having a muddy, almost a slurry, bottom, an OBS unit may sink into the bottom and retrieval may be hampered by the suction force exerted by the mud. An ocean bottom seismometer, being used to collect seismic data, must be firmly coupled to the earth. Since it is being used on the bottom below the surface of the ocean, it must weigh more than the water which its volume displaces. Furthermore, it must have a significant subsurface weight to resist the forces of ocean bottom currents. A slurry mud bottom does not provide a hard surface upon which the ocean bottom seismometer may rest. As a result, an OBS unit sinks into the mud to a point of density equilibrium. Where the density of the medium around the ocean bottom seismometer is equal to that of the unit itself. When an OBS unit is to be retrieved, the thick mud slurry may exert a suction force preventing a rise to the ocean surface.
In prior art, a simple release of ballast has been used to increase the ocean bottom seismometer's buoyancy to return to the surface. These methods have not always been successful; not only for mud problems, but also due to possible release mechanism failure.